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-   -   My goal is to get 25+ mpg from my 99 XJ with 30 x 9.5 R15 ATs (

Charley3 07-21-2013 02:32 PM

My goal is 25+ mpg in my 99 XJ with 30 x 9.5 R15 ATs, 3.55 diff gears, auto tranny
Preface: The preface was written after this thread already existed.
I am not a mechanic. I don't claim to be an expert. I am an enthusiastic amateur with 14 years experience on three Jeeps with 4L engines. I experimented with improving gas mileage on each Jeep, and I've gotten more successful with each Jeep as I learn new tricks. I have done many things (documented in this thread) as an experiment and hobby to increase my 99 Jeep XJ fuel economy as much as feasible (with my modest mod budget). I'm sharing this info so that it may be of use to others, or at least interesting to some.

In addition to my ideas there are many excellent ideas contributed by others in this thread. Many of the ideas have been implemented successfully, some haven't been implemented, and one failed but was a good learning experience. The details are are documented in this thread. This thread is a successful community project and is ongoing. I appreciate the economodding ideas and advice of others and I sometimes implement their ideas.

If you decide to participate and try some of these mods, you will be participating in a fun project that might benefit your gas mileage, but I cannot promise specific results because your Jeep is different that mine with more or less miles and wear on it, more of less lift, tire size, or weight, etc.

My gas mileage has gone from 15 to 25 mpg at 60 mph on level highway with minimal cost in parts, and I'm not finished yet. I still have more mods I want to do, but I have done most of the low cost mods already. If I do more mods it will begin to get expensive. I am currently saving $96 a month in gas (after spending $790 on parts). It took me 2 years of experimentation with my current Jeep to get to this point. I might can help you short cut the process by following my recipe so you don't have to experiment as much. However, it should still be viewed as your experiment, not an investment, since exact results cannot be guaranteed. It didn't require much money at all for me to get to 23 mpg.

Keep in mind that I have a lift, 30" tires, and 200 lbs of skid plates. If your XJ is stock height, has 225 or 235 tires, and no skid plates, it's possible you might exceed my gas mileage.

Weather (temp and humidity) affects gas mileage quite a bit. So all tests should be done in similar weather so you can evaluate the effects of each mod. I like to do most of my gas mileage tests during warm dry Summer days when temp is between 75F to 85F (typical Summer days in my climate). I also run Winter tests, but I fully expect less gas mileage during Winter because it's colder. Also be aware that increased humidity reduced gas mileage because your car has to push through more water molecules in the air, which creates more drag. Rain or wet pavement reduces gas mileage because it increases drag.

Many/most mods help in any climate or season, but some are for Summer in hot climates, and some are for Winter in cold climates. I run separate Summer and Winter tests because of these reasons, and because it's a good idea to find out how climate affects gas mileage.
For examples: Hood vent(s) are intended to help during hot Summer conditions. A grill cover is intended to help in Winter conditions. Since my climate varies from moderate to cold, I do more of the Winter mods and less of the Summer mods than people in hot climates do.

Warning: Do one mod at a time, then test (for at least 2 weeks) and study results before doing another mod.
After each mod: Reset computer's adaptive memory. Then drive it for 2 to 4 weeks (while computer relearns), then retest gas mileage (in same weather conditions as prior test). This way you learn how that mod affects performance and gas mileage. If it helped, you can know how much it helped. If something went wrong and it didn't help, or if it caused a reduction in performance or gas mileage, it's easy to diagnose which mod is the problem because you only did one mod during that test period.

If you do multiple mods in a test period (more than one mod at a time) you won't be able to know how much improvement came from each mod, and if there's a problem it's difficult to diagnose.

Only do one mod per test period!

Instructions for resetting computer adaptive memory can be found later in this thread, and many other threads on this forum and other forums (and yes there is more than one correct way to reset it).

Beginning of original OP starts here:

When I bought my XJ, it was getting 15 mpg highway, even though my XJ showed no codes and ran reasonably smooth.

Plugs, cap, rotor, and plug wires needed replacing. Did those things. Those things got me from 14 to 16 mpg highway. A little improvement, but not much. I was disappointed.
Edited in Later: I wish I had used the Kolak tune-up kit aka ignition upgrade kit for Jeep 4L engines. I think it sounds excellent and would be a great set of tune-up parts that would help gas mileage and power for sure, and might even help more than other brands of tune-up parts would. See post 1032 and 1993 for more info about Kolak tune-up/ignition kit and where to buy. Around $230 including shipping (includes upgraded MSD coil, upgraded plug wires, upgraded plugs, and quality cap/rotor with brass contacts).

Edited in Later: I bought Kolak tuneup/ignition kit. It's excellent. Gap plugs to 0.045. My engine starts easier, runs smoother, has more lowend torque and high rpm HP, and it helped my gas mileage a little. The improvements are especially noticeable in cold weather.

That is comparing to the brand new Oreilly tuneup parts I had before, which were of high quality, but no where near the quality or performance of Kolak kit.
Then Motorvac, new NGK or NTK oxygen sensor, and 205F Stant thermostat. Those 3 changes were done on same day. So I don't know how much each thing helped, but the combination improved my gas mileage substantially.
Motorvac is a top-end engine cleaning treatment done at mechanic shops where they use a Motorvac machine/pump to pump (under pressure) Motorvac cleaner through top-end of engine to clean out carbon deposits. Works much better than Seafoam.

After Motorvac cleaning, change motor oil (just in case some of cleaner got into bottom-end of engine and contaminated oil).
This is why I waited until my motor oil was due to be changed anyway, then did the Motorvac, then changed motor oil.

On an LJ I used to have, it gained 1 to 2 mpg (depending on weather/season) from 205F thermostat. So I think that's a good indicator of the 205F stat on a 4L engine (in my moderate climate).


Edited in Later: As discussed later in this thread:
See post 1125. for more info about thermostats, including which I recommend for each climate, brands, part numbers, where to buy from, etc.

A 205F stat is helpful in moderate climates to cold climates because it allows engine to run at (or near) full 210F op temp year round (instead of under-heating at 185F to 195F). IME, this is a big help in a Northern climate and does improve gas mileage noticeably. Over the last 20 years, I've done this on 3 Jeeps and 2 cars in a climate that varies from moderate to cold. It also works fine here in Summer, but we usually have 70s to 80s days in Summer, and sometimes low 90s. Very occasionally it hits 100F here in Summer. I find the 205F stat helps my a little in on cooler Summer days, more help in Spring and Fall, and a great help in Winter. On our occasional hot Summer days it is no help, but doesn't hurt anything.

If you live in a frequently hot climate your engine is already at, or near, full op temp with stock 195F stat. If you live in a hot climate, a 205F stat offers no benefit and might be a detriment. If you're in a hot climate, use stock Mopar 195F stat or upgrade to Stant 195F Superstat. They open at stated 195F temp. If your stat is to hot for your climate, your fans work harder, which reduces gas mileage. It takes power to turn a mechanical fan, especially when fan clutch engaged. It takes power to turn an electric fan. Turning fans creates parasitic drag on engine.
Edited in Later: Hoodvent(s): A great mod for people in hot climates is a hood vent(s) located on the front third of the hood. Ideally approx right above the frontend of engine. You don't want them near the windshield because high pressure air from windshield wouldn't allow enough air to exit the vents. The beauty of vents is they cool the engine compartment without using any power to do so. It's free cooling, which in theory will save gas mileage because your fans won't need to work as hard. I haven't personally tested this mod though because I don't live in a hot climate and I don't need or want this mod in my moderate to cold climate. The ideal hood vents would be ones that can be opened in Summer and closed in Winter.
The ideal situation is to keep engine as hot as possible without triggering the fans. 208F to 213F is ideal for this since the fans trigger around 215F. The ideal temp (for Jeep 4L engines) is 210F. To accomplish this, people in cooler climates need a 205F stat, while people hotter climates (regularly 95F+) need a 195F stat and hoodvents.
My XJ (with 205F stat) runs 208F most of the year. On exceptionally hot days my XJ runs 210F. My fans trigger at 215F. So my fans seldom come on, except when off road on hot Summer days. Keep in mind I am in a Northwest USA climate that is cool most of the year, and a hot Summer day here is 90F to 95F at most, except for the occasional 100F day.

When weather occasionally gets above 95F here, my 205F stat is fine for protecting my engine from overheating because my stock fans never let engine get above 215F (engine damage doesn't occur until above 230F). However, my gas mileage suffers when fans run all the time on those occasional 95F+ days, but in this area we only get 0 to 5 days a year that are 95F or hotter.
Replacing mechanical fan with electric fan is a great efficiency improvement mod in a moderate to cold climate (because fan seldom needed and seldom comes on), but offers no efficiency benefit in a hot climate (where fans have to run often).
Edited in Later: The downside of converting the mechanical fan to electric is it's expensive to hire it done, and complicated to do yourself. The biggest complications are getting it to fit, wiring it properly, and figuring out an appropriate and reliable temperature sensor to automatically and reliably trigger it. For these reasons, I might not do this mod, and I recommend caution to others considering this mod. A lot of people have had difficulty getting this mod done and ongoing difficulty getting it to function properly. I know a local radiator shop that would do this mod for me for $500 to $750, incl parts and labor, and it'd probably be reliable to; but it's not worth the expense me at this time.

So although replacing mechanical fan with electric is good in theory, but in actual practice it's not easy or affordable to get working properly. If you do this mod, it should be the LAST mod you do because there are many simpler, lower cost mods that will improve gas mileage.

Recently my cousin installed an AFE "Cold" (actually slightly cooler air, high volume) Air Intake and I have noticed slightly more power and highway gas mileage increased almost 1 mpg. It's a great high flow air intake, but not it's not true cold air. It's warm air drawn from the same place as the stock air intake. However, high flow warm air is good for gas mileage in a moderate climate, or a climate with seasons. I bought the dry filter for it because I don't like oiled air filters (I fear oiled filters might dirty sensors). See post 151 for more info on types of air intakes.
This approx 1 mpg improvement is compared to the K&N air filter I had in stock air box. So that'd be around 1.5 mpg improvement compared to a stock filter in stock box. IMO.
I've been reading about guys installing modern 4 hole fuel injectors from Dodge, Ford, Bosch, and others. I want to try some of those more efficient injectors. See post 210 for injector(s) part number(s) for your year 4L.

Edited in Later: I installed Dodge/Bosch 789 injectors and love them. Bought them from Motorman on Ebay for $190 including shipping. My engine starts easier, runs smoother, has a little more lowend torque and a little more highend HP performance, and improved my city and highway gas mileage a little bit.
I'm comparing my new 789 injectors to my low mileage stock injectors that had only 35K miles on them and were cleaned recently. 789 injectors outperform new(ish) good condition stock injectors. Your Jeep might need 789 or 703 or other. See post 210 for injectors part number(s) for your year 4L.

Edited in Later: I rewrote the exhaust section to replace my earlier theories with real world results after trying and testing various exhaust setups.

The Dynomax brand cat back exhaust is good breathing, easy to install, quality product, low cost, quiet, and reasonably rust resistant. I tried both the 2.25" and 2.5" pipe diameter versions to see which works best. With stock header and stock cat (low budgetm approach) a 2.25" cat back works best for lowend torque, general performance, and gas mileage. With an aftermarket Try-Y header (such as Thorley HDX-634Y) (a higher budget, higher performance solution) the 2.5" cat back (with Magnaflow 2.5 in/out high flow cat) works best. When I say "best" I mean of the options tried by myself and another person in this thread (Christianbe). Between the two of us we tried several exhaust combinations. I tried 2 on my earlier Jeep, 3 on my current Jeep, and he tried 2 or 3 on his current Jeep.
Edited in Later (after using both diameters of CB with stock header on my XJ): The 2.25 cat back improved my seat of pants performance at mid and high rpm compared to stock exhaust, and lost nothing at low rpm compared to stock exhaust. The 2.25 cat back improved my highway gas mileage 1.5 mpg, which is pretty good considering it cost only $150 incl shipping and substantially improved my performance. I can much more easily drive up mountain passes nice and fast now and easily pass cars doing so.
I didn't test city gas mileage, but based on seat of pants feel taking off from a stop, I think stock exhaust is best for city mpg (least gas pedal pressure to get moving), and 2.25 cat back almost as good. The 2.5 cat back took a lot more gas pedal to get moving from a stop - so I assume the 2.5 cat back got the worst city gas mileage.
For a low cost, effective improvement to performance and gas mileage, I recommend Dynomax 2.25 cat back system (with stock header and stock cat, assuming they are functioning properly - no leaks or clogs). The Dynomax 2.25 cat back system (part# 17340, costs around $150 including shipping) from or, or anywhere you get a good price. it's a great price and easy install for 93 to 01 XJ.
For somewhat higher cost with even greater improvements to performance and an untested amount of gas mileage improvement (I'd bet it improves gas mileage), I recommend Thorley Header (part# HDX-634Y, around $515 from Kolak or others with good price), Magnaflow High Flow 2.5" in/out Universal Cat (part# 99006HM, around $80 from Kolak, or others with good price), and Dynomax 2.5" cat back system (part# 17463, $107 from or others with good price). Prices include shipping.
Note for CA and NY: Magnaflow High Flow Cat 2.5" in/out (Magnaflow calls it Universal Cat) part 99006HM is a "Federal" cat for use in 48 states, but not in CA and maybe not NY. The high flow cat for CA has a different part number (doesn't flow quite as well, but flows pretty good). NY might have a different Universal cat part# too, not sure about NY. You can look up CA and possibly NY part# yourself at Magnaflow's website by model and year of your Jeep.
Don't worry about which model years of XJ that Dynomax' website says fit each of their XJ cat back systems. Either system will fit any year XJ. Dynomax cat back system 17340 is a 2.25" cat back. Dynomax cat back system 17340 is a 2.25" cat back. The 2.25 CB is excellent with stock header and cat. The 2.5 CB is even more excellent when combined with Thorley header and Magnaflow High Flow cat. Do NOT combine the 2.5 CB with stock header (that combination gives weak performance and does not help gas mileage. I installed and tested it.).

I'm not theorizing. I tried both 2.25 CB and 2.5 CB with stock header (and a few other exhaust setups/combinations). Christianbe (participant in this thread) is using Dynomax 2.5 CB with Thorley header and Magnaflow High Flow cat. So the exhaust advice here isn't theory or guessing. We've installed and tested these things IRL.

The only interesting exhaust setup combination we didn't test is Thorley header with Magnaflow 2.5 in/out high flow cat and 2.25" Dynomax cat back. I plan to install and test this combination in future.

For the very best and most rust resistant 2.5" cat back exhaust system I'd recommend the Kolak cat back exhaust or a Thorley 2.5" cat back exhaust. Either is the caviar of cat backs, IMO. Both are high grade stainless. The Kolak exhaust pipe is made by Mike Leach (of Leach header fame). Those are (IMO) the best 2.5" cat backs. They cost several times more than a 2.5" Dynomax does. The performance of the Kolak/Leach and Thorley cat backs is same or very similar as Dynomax. The advantage of the Kolak/Leach and Thorley cat backs is they are very rust resistant and would probably last longer than your Jeep. The Dynomax is aluminized steel and is rust resistant too, but not in the same league of rust resistance. The Dynomax is plenty good if you don't live where roads are salted. Even if you do live where roads are salted, a Dynomax would probably last 5 years and it only costs $107 incl shipping.

For the best 2.25" cat back, the Dynomax is probably as good as it gets, and that's pretty darn good. It's only $150 incl shipping.

For more cat and cat back exhaust info see post 2512.

Downpipe aka Crossover Pipe:
Exhaust discussion should include the downpipe. This is the pipe that connects the header to the cat.

The stock downpipe (crossover pipe) from header/manifold to cat has a huge dent that restricts exhaust. I plan to replace with a dent-free downpipe. This cheap mod will help power and possibly gas mileage when a high flow cat back exhaust is installed.

Edited in Later: For $80 installed my local exhaust shop made me a 2.5" non-mandrel bent dent-free downipe made of mild steel that works great (after a few fitment/clearance adjustments explained in following paragraph). It would be even nicer if it was mandrel bent and made of stainless steel, but what I got works great and only cost $80.
Warning: Make sure to tell your exhaust shop that stock routing for dent free downpipe might not clear the driver side UCA or diff output yoke during suspension uptravel or articulation if you have no bump-stop extension. Figure out a new/better routing that will clear UCA and need locate oxygen sensor where it's not above front differential yoke (so it doesn't get hit during suspension uptravel). I learned the hard way, but some free simple adjustments saved the day for me, and it's now working great for me, even though I have stock routing and no bump-stop extension.
Edited in Later: Although my dent-free downpipe follows stock routing, and I have 1.25" lift with no bumpstop extension, I have no clearance issues with UCA or diff output yoke (after my local exhaust shop made two minor adjustments).

Insulating Exhaust:
This is an interesting topic in itself, and is worth looking. It improves performance and might improve gas mileage too. I plan to test this on my XJ. See post 2544.

Insulating Air Intake:

It's helpful for efficiency and a cheap DIY mod to insulate your air intake box, or in the case of my aftermarket AFE cold air intake insulate the heat shield. Also insulate the intake tube.

Use stick on heat shields to insulate the airbox or filter heat shield, and use teflon header wrap to insulate the intake tube. This mod slightly improves low end torque from a stop and at speeds under 25 mph. I got the idea from Dino Savva's mod diary. ( has a good selection of stick on heat shields and teflon header wrap. Most local auto stores have a poor selection.) Buy more header wrap than you think you'll need. It always requires more than you thought. I think around 15 to 20 ft is enough for intake tube. So buy 20+ ft to be sure. It's better to have to much than run out before finished.

Chips and Tuners:
Edited in Later: Chips and store bought tuners have a bad rep on this forum and every forum I've seen because they either don't work or sometimes even cause problems. This is covered in a discussion somewhere later in this thread. When I have time, I'll try to add the post #'s here where that is discussed by several JF members.
However, custom tuner FlyinRyan has a great reputation at Jeep forums for getting great results improving performance on stock and modified 4L, and strokers. Most people go to him for performance boosts and he delivers, often while maintaining full gas mileage (many reviewers say). I wonder what he could do for gas mileage if that was the primary goal? See post 2513 for more info.

Winter Mods:
205F thermostat

As described above... The 205F thermostat also helps during Fall and Spring too, if you're in a moderate or cold climate. It won't hurt anything in Summer, if you're in a moderate or cool climate. If you're in a hot climate, skip this mod. See full explanations above.

Grill Cover:
I want to make or buy an adjustable grill cover for the colder months. Like the coldest 4 months of the year. Adjustable grill covers help gas mileage 2 ways. A grill cover helps engine reach full op temp in cooler weather, and they improve aerodynamics by keeping unnecessary air from entering engine compartment, thus reducing drag.

Edited in Later: My Winter climate isn't cold enough to ever need to completely cover my grill. So a full grill cover is unnecessary and unwanted in my slightly cold Winters. My engine would overheat with a full grill cover. However, with no grill cover at all, my engine underheats during Winter. I'm going to make a partial grill cover that covers the middle 3 openings in my XJ grill. That is what many people in moderate climates have done. I'm going to follow in their footsteps.
A full grill cover is only for artic Winter conditions, which I don't have.
Edited in Later: I figured out a plan to make an aerodynamic grill cover. Will start work on that soon. It will cover middle 3 grill openings, and it's only adjustment will be removal when weather warms up. 10-6-2014

Stop Defroster from Triggering Engine's Electric Fan:
The defroster (or defroster and heater combination) triggers AC compressor to cycle on whenever the AC pressure drops below op pressure. Why is AC compressor needed at all for defroster? It helps dry (dehumidify) the defroster air to help defog window. I've been told it only does this above 40F (not sure). I'd like to leave this this part of the system working as is for better defroster performance and because keeping gas/fluid circulated in AC keeps the seals good longer.

However, every time the AC compressor cycles on, the engine's electric fan cycles on. This is inefficient because it requires power to run the fan, and in cold weather the fan makes engine take longer to warmup during city driving or rural road driving, and it prevents engine from warming up as much as it otherwise would, especially driving a lower speeds (city or rural roads) in Winter. This wastes gas by using power to turn a fan that's not needed, and the colder engine runs richer and burns gas less efficiently.

So what I wanted to do was leave the AC compressor functioning normally when using defroster (as described above), but prevent it from turning on the engine efan. However, I haven't yet figured out how to do that.

What I have figured out is this: My AC system is shot. It's out of AC gas and has one or more leaky seals. So I'm just going to unplug the electric wire that goes to the AC compressor. This will (allegedly) prevent the compressor from coming on and will prevent it from triggering the efan. If this works, I expect it will help my Fall, Winter, and Spring gas mileage be higher. They still won't be as high as Summer gas mileage, but will be at least closer to what Summer mpg is.
Note: The modern 4 hole fuel injector mod and the Kolak ignition mod will (in my personal experience) help engine fuel economy and performance (torque & power) in all seasons and climates, but those two mods are especially helpful in cold weather.

Winter gas mileage will never be as high as Summer gas mileage, but with the mods described above, Winter mpg (and performance) will be as high as possible.


Does anyone have other gas mileage suggestions? If so, please post them.


Edited in Later: I am using all synthetic oils in my XJ. This is mostly to protect engine and auto tranny from heat and wear, but also to increase gas mileage. All synthetic oils in all areas increases gas mileage an estimated 1 mpg, and reduces wear substantially.

0-40 motor oil is excellent/ideal in 4L Jeep engines for use year round in all climates.
Mobil One 0-40 is what I'm using (although I'd be just as happy with Lucas or Valvoline 0-40).
Mobil One, Lucas, and Valvoline 0-40 are excellent. Valvoline 0-40 is widely used in Europe, but very difficult to find in USA.
Synthetic auto transmission oil in my auto transmission, and 242 transfer case.
Mobil One and Valvoline synthetic auto transmission fluids are my preference.
231 transfer case uses 75-90 or 90 gear oil. I prefer Mobil One or Valvoline synthetic gear oils.

Mobil One and Valvoline synthetic gear oils are great for differentials and for a 231 transfer case, BUT do NOT use these gear oils in a manual transmission because their limited slip additives makes them to slippery for syncromesh shifting to work properly.

Do NOT use gear oil with limited slip additives in a manual transmission. In a manual transmission, only use gear oil that does NOT contain limited slip additive.

75-90 synthetic gear oil front diff (I recommend Mobil One or Valvoline)

75-90 or 75-140 synthetic gear oil rear diff (choose viscosity according to climate and severity of use (towing requires 75-140 or 80-140)) (I recommend Mobil One or Valvoline)

I like Valvoline best in 75-90, and Mobil One best in 75-140 based looking at their spec sheets and their viscosities cold vs hot, and their flash points.

Edited in Later: I have perfect alignment and perfectly balanced tires, which helps gas mileage.

My tire pressure is on the low end of acceptable to soften ride for my bad back. This is NOT helping my gas mileage. I am going to install softer springs to soften ride. Then I will start running more tire pressure, which will gain me at least another 1 or 2 mpg and put me at, or very near, 25 mpg.

I've already increased to 23 mpg from fine tuning and a few of the mods mentioned above. I still have many more mods to do. l'm sure I will get substantially more than 25 mpg when I am finished.

Edited in Later: My XJ has auto tranny, 3.55 diff gears, and 30x9.5R15 AT tires. Later in thread a member with same setup regeared to 3.73 diff gears and his gas mileage improved.

Edited in Later: Later in thread it was pointed out that my auto tranny puts me at some disadvantage for gas mileage. The manual tranny XJ have a built in advanatage for gas mileage.

I have reached 25 mpg with my auto tranny, but if I had a manual tranny I could do even better. Manual tranny 4L XJs are known to get 24 to 25 mpg highway when completely stock. So with economods a manual tranny XJ could potentially get 27 or 28+ mpg at 60 mph.

Edited in Later: I got 24.89 mpg at 60 mph on level highway in test today (Sept 13, 2014). That's pretty much 25 mpg. I still have a few mods to do. I'm hopeful I might get it above 25 mpg in future. See posts 2477 through 2480.

Edited in Later: I got 22.7 mpg at 65 mph on level highway today in gas mileage test (Sept 19, 2014). See post 2516.

The latest gas mileage figures (above) are from completion of most of the low cost mods that are possible. I think that's about the limit of what's economically feasible. I might be able to squeeze another 1 mpg out of it on a low budget. After that, any additional improvement will get expensive and require a higher mod budget.

I could get even higher gas mileage, but it would require some expensive mods to do so. I think around $1200 of additional mods might gain another 2 mpg. I think $3200 of additional mods might gain another 3 mpg. That'd be neat as a hobby, but not cost effective as an investment to save gas money.

I've already taken it almost as far as makes sense economically, but it would be fun to take it as far as I can afford and I probably will.

98muddyjeep 07-21-2013 03:39 PM

yea i have heard about people using the 4 hole injectors from a neon to replace the stock 1 hole injector,don't know how much benefit there is, i guess there should be some as the spray will be finer and burn better than a drip.

JMH21814 07-21-2013 04:21 PM

You've piqued my interest. I'll follow this post with anticipation. Good luck!

TheBlueDevil 07-21-2013 04:38 PM

you should certainly look into a bored throttle body, not a straight through muffler but one with a little more free flow, ditch anything that sticks out on top of your roof (roof racks), certainly run dual elec. fans, a new coil (accel makes a great one) and definitely do the injector upgrade.

Newtons3 07-21-2013 04:42 PM

Instead of CAI you might look at hot air. This is the way the hypermilers get the computer to lean out the fuel mix even more. I dont find the 4 hole injectors are worth anything over and above quality precision single hole. Look at the flow balance between them instead and get it as close as possible. mine are within 0.3 percent of each other. Wrap your exhaust front to rear. Swap cams to a tamer grind (under .375 lift and around 180 duration at 0.050). Couple that with a 50-52mm throttle body. Swap to the late-model head with raised exhaust ports. Start closing off and smoothing the front end. Reduce the grill opening as much as possible. Belly pan to direct air helps. Run as light as possible.

mschi772 07-21-2013 04:53 PM

Go to and look-up FastPlastic he/she has done some nice aerodynamics work with his/her XJ in addition to other MPG mods. FastPlastic has also discussed some very promising ideas that he/she has not yet implemented. You'll have to do quite a lot of work to get 25+ MPG, and you'll find that a WARM air intake will be more beneficial in this regard than cold.

With grill covers, stock to small lift with lightweight belly pan for reduced undercarriage turbulence, keep the air dam, warm air intake, no armor (keep the vehicle as light as possible, roof rack delete, electric fans, quality tune-up, quality O2 sensors, stock-sized tires, no AC (even better if you have the guts to remove the AC compressor completely)... You can hope for MAYBE 25 mpg highway. I'm using 4-hole injectors, but I wouldn't call them an MPG booster.

The 4.0 can be improved a little bit, but it's not the most efficient engine, and there isn't a ton of room for improvement. The AW4 has goofy shift points that certainly don't help either, but not much can be done there other than to swap-in an AX15 manual or something. Using a 2wd XJ would also help quite a bit. The XJ's aerodynamics are the real enemy, and where the biggest gains can be had if you have the fab skills.

Newtons3 07-21-2013 05:20 PM

The TransGo kit can help with the AW4 shifting. Lose the passenger side mirror and radio antenna. Push the wheels out where the tires are flush with the fenders. Flat wheels help. Run minimal fender clearance. Pull the front driveshaft when not needed. Trade out your 300 cpi cat for the shortest one you can find with a cell count around 100 cpi. Vent the hood (especially across the rear) AND vent the fenderwells into the engine compartment. Isolate the timing (via Msd) and play with advance. Drop pulley ratio on alternator and P/S. Mod the MAP- I like the Apexi controller. This is less beneficial on OBD-II systems.

Charley3 07-21-2013 09:22 PM

I did remove rook rack and running boards. Edited in later: Later on I also removed a winch.

I am after more gas mileage and power. So my 1st priority is anything that increases both. My 2nd priority is things that improve one without losing the other.

So far I've improved both gas and fuel economy.

I'm also considering having intake and exhauat ports ported and polished, and high flow headers (insulated with ceramic paint) and exhaust.

To have a high flow muffler that's quiet, I'm thinking a stock muffler from a V8.

Newtons3 07-21-2013 09:39 PM

The headers wont help either significantly- you already have tubular exhaust. The head work helps a lot but to realize power gains you have to change cams, and there went your mileage. In other words, theres no free lunch. You can do one or the other. But 25mpg on 30s in that 4wd while increasing performance is an unrealistic expectation. You cant have your cake and eat it too.

Charley3 07-21-2013 09:40 PM

I am after more gas mileage and power. So my 1st priority is anything that increases both. My 2nd priority is things that improve one without losing the other.

So far I've improved both fuel economy and power.

I'm considering having intake and exhauat ports ported and polished, and JBA high flow headers (insulated with ceramic paint) and high flow exhaust. I think that would gain some more fuel economy and power.

To have a high flow muffler that's quiet, I'm thinking a stock muffler from a 300 cu or 5L V8 would be quiet and high flow on a 4L.

I'm using mid grade gas. That was the case before and after the mods I've done so far.

I think I'm going to have the fuel map sensor cleaned or replaced next.

What about EGR valve? Do 99 4L have that? Would it affect fuel economy?

Newtons3 07-21-2013 09:56 PM


Originally Posted by Charley3 (Post 15703386)
I did remove rook rack and running boards.

I am after more gas mileage and power. So my 1st priority is anything that increases both. My 2nd priority is things that improve one without losing the other.

So far I've improved both fuel economy and power.

I'm considering having intake and exhauat ports ported and polished, and JBA high flow headers (insulated with ceramic paint) and high flow exhaust. I think that would gain some more fuel economy and power.

To have a high flow muffler that's quiet, I'm thinking a stock muffler from a 300 cu or 5L V8 would be quiet and high flow on a 4L.

I'm using mid grade gas. That was the case before and after the mods I've done so far.

I think I'm going to have the fuel map sensor cleaned or replaced next.

What about EGR valve? Do 99 4L have that? Would it affect fuel economy?

A stock muffler will be a waste of time if you are looking for better flow. You will need a performance type.

There is no EGR on your 4.0l.

What on your map sensor are you going to clean? Until the much later years in the Grands and Wranglers where they were mounted on the throttle body, the map sensors sit on the firewall and gets manifold signal via a hose. They generally don't get dirty as long as the tiny reference hole isn't plugged.

Mid grade gas, unless need to counteract preignition or detonation is not beneficial. Within the same type, style, brand etc., lower octane fuel has more btu energy and will achieve more horsepower and mileage than a higher octane rating.

mschi772 07-21-2013 10:13 PM

Indeed, 87 octane is all you need. Save your money.

MoparMech 07-21-2013 10:15 PM

It's a Jeep......if you want good mileage get a Dart.

Charley3 07-21-2013 10:24 PM

I just got home from a road trip.

I gained another 1 mpg from high cooler flow air intake, and noticeably more power on hills.

So now 22 mpg highway. Pretty good for 30 x 9.5 R15 all terrain tires.

I will switch to 87 octane gas and see how it does. Could be a real economy boost considering cheaper per gallon.

Thanks for suggestions.

Charley3 07-21-2013 10:26 PM


Originally Posted by Newtons3 (Post 15703462)
A stock muffler will be a waste of time if you are looking for better flow. You will need a performance type.

I meant a muffler from a V8. That'd be high flow on a 4L wouldn't hit?

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